Method for finishing plastic compositions



Patented June 10, 1947 METHOD FOR FINISHING PLASTIC COMPOSITIONS Richard H. Hunt, J12, Springfield, Mass., assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Application November 28, 1944, Serial N0. 565,575

4 Claims.

This invention relates to a method for finishing sheets and other objects of organic derivatives of cellulose. More particularly, the invention relates to a method for completely and permanently removing surface imperfections from sheets of cellulose organic derivative compositions.

One method for producing sheets of cellulose organic derivatives comprises the steps of (1) mixing the cellulose derivative with solvents and plasticizers, as desired, in a dough mixer, (2) filtering the plastic mass thus obtained to remove extraneous and foreign matter, (3) masticating the filtered mass, together with pigments, fillers and colors as desired, on a pair of rolls rotating towards each other at equal peripheral speeds to homogenize the mass and remove excess solvent, (4) baking the rolled material to consolidate the composition and convert it into a solid block, (5) skiving sheets from said block, (6) seasoning the sheets to remove all the remaining solvent and (7 finishing the seasoned sheets between opposing platens under heat and pressure. The finished sheets are sold to customers who generally reshape them to their needs by heat ing them until they are soft enough to bend or otherwise change shape under pressure, (about 180 F. to about 240 F.)

One of the major problems confronting manufacturers of thermoplastic sheet materials is the elimination in the finishing operation of surfaceimperfections which occur through accident or in the-course of the operational steps prior to the finishing step. The most difficult of these imperfections to remove completely and permanently are the sheeter lines which are caused by minute serrations in the cutting edges of the knives used to skive the sheets from the pressed block. In the standard finishing operation, only enough heat to soften the surface and permit a superficial ironing of the surface can be used since, at higher temperatures, the entire sheet flows and loses shape. The superficial ironing does not remove the sheeter lines but only glosses them over with a thin film of thermoplastic which is under strain as evidenced by the fact that reheating the sheet to forming temperatures relaxes the coating and allows the sheeter lines to become visible.

It is known that the surfaces of cellulose organic derivative sheets may be treated with a solvent prior to thefinishing step to soften the sheets so that lower temperatures and pressures may be used in the finishing operation and so that the time consumed during the said finishing operation may be materially shortened. However, none of the known treating methods has resulted in a finished sheet permanently free from sheeter lines.

It is an object of this invention to remove sheeter lines and other imperfections from the surfaces of sheets and other articles made from cellulose organic derivative compositions;

It is a further object of this invention to treat cellulose organic derivative sheets prior to the finishing process so that said finishing process may permanently remove sheeter lines and other surface imperfections.

It has been discovered that the treatment of cellulose organic derivative sheets with a solvent mixture'comprising one of the group consisting of acetone and methyl ethyl ketone and one of the group consisting of ethyl lactate and dioxane permits the permanent removal of surface imperfections and sheeter lines in the normal finishing process.

The following examples are given in illustration of the process of this invention and areL not to be construed as limitative thereof. Where parts are mentioned, they are parts by weight.

Example I A plasticized cellulose acetate sheet made by the block and sheeter method, surfaces of which contained numerous sheeter lines, was dipped in a solution made up of 80 parts of acetone and 20 parts of ethyl lactate. Excess solvent was removed by drying at 90 F. for one hour. sheet was then placed between highly polished platens and subjected to .200 pounds per square inch pressure at 275 F. for minutes. It was then cooled and removed from the press. When placed in boiling water the sheet softened until it could be bent or formed but none of the sheeter lines reappeared.

A similar sheet of cellulose acetate was polished under the same temperature and pressure conditions, but without having been subjected to the solvent dip. When this sheet was placed in boiling water until it was soft enough to reshape, a majority of the sheeter lines reappeared.

Example II A sheet of cellulose acetate having a plurality of sheeter lines on its broad surfaces was dipped in a solvent containing parts of methyl ethyl ketone and 20 parts of dioxane. After the excess solvent was removed, the sheet was polished under conditions similar to those of Example I and the finished sheet was found to be capable of The being heated to forming temperatures without reappearance of sheeter lines and other surface imperfections previously present.

Example III A sheet of cellulose acetobutyrate made by the sheeter method was treated with a solution containing 95 parts of acetone and 5 parts of ethyl lactate. Excess solvent was removed from the sheet and it was then pressed between highly polished platens at 240 F. and under 150 pounds per square inch pressure. A strip 12" x 2" from the resultant sheet was heated to 200 F. and then bent through an angle of 180 without the reappearance of sheeter lines.

Example IV A sheet of plasticized ethyl cellulose made by the sheeter process was treated with a solution containing 60 parts acetone and 40 parts dloxane. The resultant sheet was polished after the removal of excess solvent under the same conditions as in Example III and no recurrence of sheeter lines was observed when the polished sheet was reheated to forming temperatures.

The amount of either component of the solvent mixture of this invention may be varied con.- siderably. Thus, the acetone or methyl ethyl ketone may be present in amounts ranging from 60 to 95 parts by weight and the amount of ethyl lactate qr dioxane may vary between 40 and 5 parts by weight.

The cellulose organic derivative sheets which may be treated by the process of this invention are plasticized or unplasticized sheets of cellulose esters, e. g., cellulose formate, cellulose acetate, cellulose propionate, cellulose butyrate, cellulose mixed esters, e. g., cellulose acetopropionate and cellulose acetobutyrate and cellulose ethers, e. g., methyl, ethyl, and benzyl cellulose. The process of this invention is particularly applicable to sheets of cellulose organic derivatives made by the block-press-sheeter process described above, but it may also be used to refinish formed objects which may be finished in a mold under heat and pressure after being treated with the solvent mixture. For example, a compression molded article of cellulose acetate, the surface of which has become inadvertently marred, may be treated with the solvent mixture of this invention and then refinished under heat and pressure in the original mold.

The solvent mixture of this invention may be applied to the surface to be treated by any of the known methods such as sp y brushing roll coating the article, or by dipping the entire article in a bath of the solvent mixture.

The finish imparted to the articles after action of the solvent mixture may be a high gloss or dull matte or any embossed design, as desired.- The finish is imparted by the nature of the surface of the opposing platens in the finishing operations and these surfaces may be altered at will to give the desired results.

Sheets prepared by the process of this invention may be most advantageously used in the fabrication of transparent closures for airplane cockpits. These closures are made by shaping cellulose organic derivative sheets under pressure after heating them to about 180 to 240 F. in a heated fluid such as water or glycerin or in heated air such as in an oven. Sheets made by prior processes have proven of little value due to the recurrence of sheeter lines and other surface imperfections with consequent optical aberrations which make them unfit for the intended use. Sheets made by the process of this invention may be formed into the complex shapes of cockpit enclosures without danger of spoilage due to the return of surface imperfections.

The process is also of advantage in reclaiming accidentally marred sheets or articles requiring a, high lustrous finish.

The foregoing description is intended to be descriptive and not limitative of'the invention as described in the appended claims.

What is claimed is:

1. A method of permanently removing surface imperfections from articles of cellulose acetate which comprises softening the surfaces of said articles by treatment with a solvent mixture comprising 60 to parts by weight of one of a group consisting of acetone and methyl ethyl ketone and 40 to 5 parts by weight of one of a group consisting of ethyl lactate and dioxane, removing only excess solvent and thereafter finishing said articles under heat and pressure while maintaining the surfaces of said articles in a softened condition.

2. A method of permanently removing surface imperfections from articles of cellulose acetate which comprises softening th surfaces of said articles by treatment with a solvent mixture comprising 60 to 95 parts by weight of acetone and 40 to 5 parts by weight of ethyl lactate, removing only excess solvent and thereafter finishing said articles under heat and pressure while maintaining the surfaces of said articles in a softened condition. i

3. A method of permanently removing surface imperfections from articles of cellulose acetate which comprises softening the surfaces of said articles by treatment with a solvent mixture comprising 60 to 95 parts by weight of acetone and 40 to 5 parts by Weight of dioxane, removing only excess solvent and thereafter finishing said articles under heat and pressure while maintaining the surfaces of said articles in a softened condition.

4. A method of permanently removing surface imperfections from articles of cellulose acetate which comprises softening the surfaces of said articles by treating with a solvent mixture comprising 60 to 95 parts by weight of methyl ethyl ketone and 40 to 5 parts by weight of ethyl lactate, removing only excess solvent and thereafter finishing said articles under heat and pressure while maintaining the surfaces of said articles in a softened condition. RICHARD H. HUNT, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,657,870 Sherts Jan. 31, 1928 1,889,061 Dickie et al Nov. 29, 1932 1,996,102 Dreyfus Apr. 2, 1935 2,117,209 Pitman May 10, 1938 2,294,479 Peter Sept, 1, 1942 

